The following guest post is by Tom Clark and Sanford Gordon, professors of political science at Emory University and New York University respectively and experts on judicial politics. — Erik Voeten

By pulling the trigger on the “nuclear option” yesterday, Senate Democrats lowered the hurdle for executive and most judicial nominations from 60 to 51 votes. The immediate trigger for this momentous development was that Senate Republicans blocked three nominees to the US Court of Appeals for the District of Columbia Circuit. (This particular court is especially important because of the role it plays in reviewing executive action, which will likely be an important area of adjudication as President Obama seeks to circumvent congressional gridlock during his last term.)

One of the key arguments Republicans have advanced for not acting faster on President Obama’s judicial nominations – particularly for the DC Circuit – is that there is no pressing need. Indeed, while we often hear concerns about a vacancy crisis on the courts, this claim is disputed, such as here and here. Indeed, critics have alleged that President Obama is engaging in “court-packing” in the spirit of FDR’s famous attempt to add additional seats to the US Supreme Court after that Court overturned multiple provisions of his New Deal plan. The most obvious alternative explanation is that Republicans are worried that President Obama’s nominees will tilt the courts too far to the left.

These amount to empirical claims, so let’s have a look at the evidence.

We begin by considering the workload for the courts of appeals, which are directly beneath the US Supreme Court and are, generally speaking, the last resort for most legal disputes. This figure shows the number of cases filed per judge in each of the 12 courts of appeals each year from 1937 through 2012. For the most recent years, we also show the number of cases filed per active judge. (We calculate the daily number of vacancies for each circuit each year and subtract the median number of vacancies from the authorized number of vacancies to estimate the effective number of active judges.) A few patterns stand out.

Graph by Tom Clark and Sandy Gordon

First, the number of cases filed per judge has risen dramatically in most circuits since 1937. Second, there is considerable variation in the number of cases filed per judge across circuit. The judges of the 11th Circuit are the most heavily-worked judges, whereas the judges on the 10th Circuit are much less worked. The Court of Appeals for the District of Columbia Circuit, though, stands out as having the least number of cases filed per judge, and having experienced the least growth in its docket. This evidence is on its face consistent with the GOP claim that there is no emergency on the DC Court of Appeals. Indeed, the difference between the dashed line and the solid line – the difference between a fully-staffed court and a court with the current number of active judges – is marginal at best. In 2012, a fully staffed court would have resulted in a decrease from 109 cases per judge to 100 cases per judge. Of course, the cases the DC Court of Appeals hears are some of the most important and complex in the country, and so even the lower workload and the difference of 9 cases per judge per year might be consequential in this instance. What is more, while the DC Circuit’s caseload is lower than the other circuits’ caseloads, this has always been the case.

An alternative take is to consider the consequences of vacancies and efforts to fill them on the partisan makeup of the courts of appeals. Nearly all cases heard by these courts are decided on by three-judge panels, where a panel’s participants are essentially randomly chosen from the current judges on the court. So a question of tremendous policy relevance is the likelihood that a three-judge panel will have a Democratic (or Republican) majority, where one might consider a judge’s partisan ship as the party of the president who appointed her. Who is a “current” judge is a bit tricky, though, since older jurists often take on a “senior judge” position, hearing fewer cases than their full-time counterparts. But we can estimate the probability that a panel has a Democratic majority under different assumptions about the participation rate of senior judges.

Graph by Tom Clark and Sanford Gordon

The solid lines in this figure display the probability of a Democratic majority in a three-judge panel in each Circuit, varying the posited senior judge participation rate from zero to 100% of the participation rate of full-time judges. A decreasing line implies that the ratio of Democrats to Republicans in the set of senior judges is lower than the ratio among full-time judges; a flat line implies a similar ratio. As is evident from the figure, there are dramatic differences in the partisan make-ups of the different courts: for example, the Fifth Circuit (encompassing Texas, Louisiana, and Mississippi) is quite Republican whatever one’s assumptions about senior judges, while the ninth (which includes California, the Pacific Northwest, and neighboring states) is heavily Democratic.

This brings us to vacancies. The dashed lines in the figure display the probability of a Democratic majority in a three-judge panel if President Obama fills all vacancies. (Panels in the figure with no dashed line correspond to Circuits with no current vacancies.) Unsurprisingly, in the heavily Republican and heavily Democratic circuits, the president gets little “bang for the buck” by filling vacancies; alternatively, Republicans have little to fear from an Obama appointment in those cases. But consider the DC Circuit – the source of the current controversy. Among full-time judges, the court is split 4-4, implying a 50% chance of a Democratic majority in a three-judge panel if senior judges sit out all cases. Fill the three vacancies with Democrats, and that probability jumps to 72%. Because five of the six senior judges in the DC Circuit are Republicans, the probability of a Democratic majority under the assumption of full participation by senior judges is just 27%. Add three Democrats and the number jumps to 45%.

Another quantity of interest is what we might call the President’s incremental leverage: the percentage point change in the probability of a Democratic majority on a panel associated with filling a single vacancy. Suppose, for example, that senior judges hear about half of the cases of their full time counterparts. In that case, filling a single vacancy on the Sixth Circuit only buys the President a 2.5 percentage point increase in the probability of a Democratic panel, while filling one on the DC Circuit buys him an 8.1 percentage point increase. Interestingly, incremental leverage is greatest for the first circuit: almost an 11 percentage point increase.

Finally, this figure directly compares the two current claims explaining strong GOP resistance to Obama’s judicial nominees. On the left we plot the average length of all current vacancies in a circuit against the circuit’s current caseload per judge (looking only at circuits with current vacancies). Average vacancy length is attributable to a number of factors, including obstruction. The dashed line is from a linear regression model that excludes the 9th circuit. (The 9th circuit is an outlier in two respects: it is more than double the size of the other circuits, so each vacancy has a much smaller consequence; and one seat has been vacant for nine years.) We see no evidence of a correlation between the length of delay and the courts’ workloads. However, the right-hand panel shows the correlation between delay and the President’s incremental leverage (as defined above). Here, we see weak evidence of a positive correlation, with higher incremental leverage associated with longer vacancies.

Graph by Tom Clark and Sandy Gordon

To summarize, the evidence suggests that while the DC Circuit does have a smaller workload (as measured by the number of cases) than other courts, this is nothing new. This, coupled with the absence of a correlation between workload and delay, suggests that efficiency was unlikely the driving force behind the obstruction that precipitated the nuclear option. Moreover, filling those vacancies would have a trivial effect on average workload on the courts. Second, we have quantified the extent to which President Obama’s ability to fill vacancies can affect the partisan make-up of three-judge panels that hear nearly all appeals. The evidence suggests that courts of appeals in which an appointment would most dramatically change that make-up tend to be associated with longer-lasting vacancies. It seems, then, that between the two concerns – judicial efficiency and partisan control of the judiciary – concerns about partisan control precipitated the yesterday’s momentous change to the Senate rules.

postscript: The original post accidentally omitted the paragraph that starts with “The solid lines”

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